Stereoscopic image sequences are preferred over standard monoscopic image sequences because they provide an additional dimension that can significantly enhance a viewing experience. Not only is perceived depth enhanced, but so is perceived sharpness. However, the advantage of stereoscopic image sequences over monoscopic sequences is obtained with a cost in bandwidth and/or processing. Without extra compression, the bandwidth required to transmit stereoscopic sequences is twice that required for monoscopic sequences, because two streams of images or video signals must be transmitted, one for the left eye and one for the right eye, compared to the single stream required in monoscopic image sequences.
It has been proposed that asymmetrical coding can be used to reduce the required bandwidth for stereoscopic imagery and video signals. One such method has been proposed by M. G. Perkins, in xe2x80x9cThe Compression of Stereoscopics, IEEE Transaction in Communications, 40, pp. 644-692, 1992, and by T. Mitsuhashi, xe2x80x9cSubjective image position in stereoscopic TV systemxe2x80x94Consideration on comfortable stereoscopic images, Human Vision, Visual Processing and Digital Display V. 2179, pp. 259-266, 1994. Therein it is suggested that by providing a high quality picture to one eye and a reduced quality picture to the other eye, the fused-picture appreciated by the eyes would have a high quality, being dominated by the higher quality picture; this phenomenon has been reported before by visual scientists, e.g., B. Julesz, in Foundations of Cyclopean Perception, University of Chicago Press, p. 96, 1971. Thus, the viewer receives, or appears to receive, a high quality picture even though one eye is seeing a reduced quality picture. However, with the above systems, particularly with respect to the system of Perkins, bandwidth reduction is obtained by showing an image having lower quality consistently to one eye. Thus, although stereoscopic images can be of acceptable perceived quality with this method, it produces an imbalance of quality between the two eyes.
As explained above, bandwidth reduction for stereoscopic video signals can be obtained by having one image of a stereoscopic pair of lower quality. Such reduction in image quality to one eye is compensated for by the image quality presented to the other eye, and the perceptually combined stereoscopic images can appear to have high quality. However, the purposeful introduction of an imbalance of quality between the eyes is not desirable for several reasons, including concerns for health, usage, and comfort. Specifically, effects of a sustained imbalance in the two eyes on children, whose visual system may still be developing, can be a safety and health concern, and its usage may give rise to potential liability for 3-D television broadcasters. An imbalance in the two eyes is also not good for people who have a strong dominance in one eye because the lower quality image may end up being presented consistently to the dominant eye, leading to perceived image quality that is worse than intended. Furthermore, this method may lead to visual fatigue especially with sustained viewing, because of a constant imbalance of image quality between the eyes.
The present invention overcomes the above disadvantages by balancing image quality between the two eyes over time. Broadly, the present invention comprises a step of interleaving high-quality images with reduced-quality images for each eye. This methodology spreads the reduced-quality images over the two eyes while maintaining benefits of perceived high image quality and savings in resources required for processing, storage, distribution, transmission, and display of stereoscopic images.
An important feature of the preferred embodiment of this invention is that the inter-leaving is controlled in such a way that reversals in image quality within a stream of images for one eye are yoked to the reversals in image quality in the stream to the other eye. Effectively, there is a cross-switch in image quality between the two streams so that high quality images are alternated between the left and right eyes; one eye always receives a high quality image. By switching, or reversing, the quality of the image between the two eyes, a more balanced effect on the eyes is obtained.
Another novel and key aspect of an embodiment of the invention is that the switching of image quality between the two streams of images occur at significant changes in image contents or features along the streams, so as to render the cross-switching unnoticeable by viewers. The switching need not occur at regular or identical periods; an important aspect of the switching in this embodiment is that it is activated only when a significant change in the picture content occurs. Significant changes are known to occur at scene changes for example, or during a high level of movement within images. Switching is optionally carried out at a significant change in picture luminance. The switching intervals typically vary considerably, eye to eye, there being an overall equalization of the time periods. By switching at significant changes in picture content, the switching is not as noticeable by the viewer, who continues to see a high quality stereoscopic images while disadvantages of possible ill effects on children, problems of a dominant eye and visual fatigue are overcome. Further, reduction in the bandwidth is also achieved. When switching occurs absent a significant change in the image content, the switch is noticeable by the viewer though the advantages of the invention remain supported.